The problem of achieving a uniform stress in all the fasteners of a flange joint or connection has been known for some time. There have been a number of studies by different investigators, and numerous patents and publications have resulted.
Typical attempts at solving the problem of obtaining uniform stress in all fasteners such as bolts of a bolted joint, involved methods of torque control with multiple passes of a predetermined tightening sequence. A "pass" in the context of bolt tightening refers to tightening of all of the bolts in a joint once. Such attempts sometimes assume that the application of essentially uniform torque to each bolt during tightening will result in essentially uniform stresses in all of the bolts in the tightened joint. For example, U.S. Pat. No. 4,969,105 is representative of tightening methods involving torque control. The patent describes a method for tightening bolts in a gasketed joint and includes a method for compensating for gasket compression, which has been found to induce the undesirable effect of relaxation or loss of fastener tension due to continued compression of a gasket after torque upon a bolt is eliminated. This method includes monitoring and control of a drive system to continuously compare a torque related variable, such that the drive system will apply pulses of torque until the application of torque causes only insignificant increases in gasket compression.
Research has found that the bolt stress for a given applied torque varies greatly due to changes in the coefficient of friction. Research has further shown that interaction between bolts and other alignment problems contribute more to the variation and final bolt loads than do problems associated with torque control. Attention is called to J. H. Bickford, "The Bolting Technology Council and the Search for More Accurate PreLoad", Advances in Bolted Joint Technology, 1989, PVP Volume 158, Honolulu, Hi. U.S.A., Jul. 23-27, 1989.
Another alternative method involves control of the rotational angle through which a bolt or the like is turned. It has been found that the tightening force or axial tension of the fastener is closely related and increases according to the angle of rotation during tightening. U.S. Pat. Nos. 4,413,396 and 4,685,050 furnish examples of methods for tightening threaded fasteners wherein the torque/rotational angle characteristics are compared and matched with theoretical values to achieve proper tightening of the fastener. Another bolt tightening procedure was described by D. H. Van Campen, "A Systematic Bolt-Tightening Procedure for Reactors Vessel Flanges", First International Conference on Pressure Vessel Technology, Sep. 29, 1969 to Oct. 2, 1969, pages 131-141, 1969. The author, working with bolted flange connections for a high pressure reactor vessel, developed a set of equations for such vessels that involved hydraulic jack pressures and bolt strain. The test reactor included a bolted flange having 36 bolts which were separated into four series, wherein the method assumed a single constant for all of the bolts in a series. Such an assumption has been found not to hold true, especially as the flange joint becomes more complex. It was further assumed that the flange was symmetrical which assumes either that the flange has no imperfections or that any such perfections are symmetrically located. A further assumption was made in that the elastic interaction between bolts of a system remain constant. The method obtained reasonable agreement between theoretical or calculated jack pressures and those actually required when tightening all bolts to a uniform low pressure, but had much greater discrepancies between theoretical and actual jack pressures when the bolts were tightened to higher pressures.
In spite of the considerable study of the problems of tightening bolts or other threaded fasteners in a joint so as to obtain substantially uniform stress on each bolt, no one heretofore has achieved a method in which all of the bolts in a joint can be tightened to essentially the same final stress after interaction in a single pass, or in a real time operation wherein the number of tightening passes is greatly reduced and joint reliability greatly increased.